In recent years, increasing attention has been directed at the noise characteristically emitted by aircraft gas turbine engines, and Federal regulations now limit the permissible noise levels. Accordingly, more effective noise suppression techniques are continually being sought by the gas turbine engine design community. One technique which has found wide-spread acceptance in reducing the noise propagating from engine inlet and exhaust ducts is to line the duct walls with a sound-suppression, or sound-absorbent, material. In one form, the material comprises a sandwich of two thin metal facing sheets or skins separated by a core material, generally of the cellular honeycomb variety. This honeycomb sandwich material has its inner skin perforated so that all the cells are vented to the duct flow path. As is well known, the cells function as Helmholtz resonators to tune out noise within a frequency band which is related to the cell size. In order to broaden the band of frequencies suppressed without increasing treatment length, a stacked configuration may be employed wherein a plurality of cellular cavities having a variety of cavity volumes are spaced from the duct by a variety of distances, with a plurality of neck passages provided for communicating between the various cavities and the duct. U.S. Pat. No. 3,819,009, Motzinger, entitled "Duct Wall Acoustic Treatment," which is assigned to the assignee of the present invention and the disclosure of which is incorporated herein by reference, is representative of such a structure.
When such a stacked sandwich material is employed for noise suppression in a hot gas environment, typified by gas turbine engine exhaust nozzles and ducts, a potential differential thermal expansion problem exists. This is due to the large temperature gradient which exists between the hot flow path defining honeycomb facing sheet and the relatively cooler opposite (backside) facing sheet. As the engine is cycled throughout its operating range, cyclic thermal stresses are imposed on the sound-suppression material. These thermal stresses and the resulting distortion and fatigue may reduce the structural life and, thus, effectively increase the cost of the engine over its life cycle.
Therefore, a means is needed for making use of the inherent acoustic advantages of honeycomb sandwich material in a hot gas environment without subjecting it to high levels of thermal stress. In short, the problem is to use the honeycomb structure so as to take advantage of its acoustic properties without incurring structural liabilities in a hot gas environment.